What size weedwhackers do you have? There are a lot of small size whackers of 25-31ccs out there. There are also some larger models, sometimes called brushcutters, that use the larger engines popular with MBers as well.
From what i've seen there are two common size clutches on small motors, 78mm and I believe 56mm on the common smaller weedwhackers and chainsaws.
The commercially made friction drives use the larger clutch because they usually suggest motors larger than 31ccs where the 78mm clutch is used.
One problem in engineering occurs after the clutch though. How will you mount the drive spindle so it aligns with the clutch and how will you support it with bearings? I think that's why many diy friction drives simply hang the motor over a wheel, bypass the clutch and direct drive the spindle off the motor shaft.